<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        li {
            list-style: none;
        }

        .box {
            width: 100px;
            text-align: center;
            margin: 100px auto;
        }

        .btn {
            width: 100px;
            height: 50px;

        }

        .box ul {
            display: none;
        }

        .box ul li {
            width: 100px;
            height: 50px;
            line-height: 50px;
            background-color: pink;
        }

        .wan {
            display: block !important;
        }
    </style>
</head>

<body>

    <div class="box"> <button class="btn">全部应用</button>
        <ul>
            <li>1</li>
            <li>2</li>
            <li>3</li>
            <li>4</li>
        </ul>
    </div>
    <script>
        var btn = document.querySelector('.btn');
        var fn = document.querySelector('ul');
        btn.onmouseover = function () {
            fn.className = 'wan';
        }
        btn.onmouseout = function () {
            fn.className = '';
        }
    </script>
</body>

</html>